1. Field of the Invention
This invention relates to measuring moisture content of a sheet, e.g., a plastic interlayer, used in the manufacture of laminated windows, e.g., structural windows and aircraft or automotive windshields.
2. Discussion of the Technical Problems
In general, in the manufacture of automotive windshields, plastic sheets, e.g., of the type taught in U.S. Pat. No. 4,101,529, are cut from a continuous web, washed, dried and thereafter positioned between a pair of glass plates. The glass plates having the plastic sheet or interlayer therebetween, are subjected to heat and pressure, e.g., as taught in U.S. Pat. Nos. 3,987,449 and 4,046,951 to form a laminated windshield. The moisture content of the interlayer can effect the quality of the finished product, for example, if the moisture content is above about 0.70%, the adhesion of the sheet to the plates is less than desired and could result in delamination. On the other hand, if the moisture content is below about 0.30% by weight, the adhesion to the plates it too great and could lower impact resistance of the windshield.
The plastic sheets are cleaned, dried and positioned between the glass plates to form a subassembly in a controlled environment and the moisture content of the plastic sheet is checked at selected times, for example, after the washing and drying operations. One moisture monitor technique is to manually check selected sheets, e.g., using a Lockwood McClorie infrared moisture analyzer. Although this technique is acceptable, it does not give moisture content of each plastic sheet and usually requires the manual steps of removing the sheet from a conveyor, measuring the moisture content of the plastic sheet and returning the plastic sheet to the conveyor.
Also available are instruments for measuring moisture content in a continuously moving plastic web. Such an instrument is sold by Moisture System Corporation as a Quadra-Beam.TM. moisture analyzer. Although this instrument is acceptable for measuring moisture content in a moving continuous web, additional considerations are required when measuring moisture content in moving spaced plastic sheets. For example, the response time of the instrument circuit when changing from a non-testing period, e.g., between trailing edge of leading plastic sheet and leading edge of trailing plastic sheet to a testing period, e.g., measuring moisture content of a plastic sheet, should be short so as to accurately and completely measure the moisture content of the upstream sheet. This limitation can be overcome by modifying the instrument circuit, by selectively changing the conveyor speed or reducing the spacing between the sheets. Although these modifications can be made, it would be advantageous to provide a system for automatically inspecting moving spaced plastic sheets that does not require modifying the instrument or the conveying equipment.